1. Field of the Invention
The present invention relates to a variable focal length lens system.
2. Related Background Art
As for recording an image of a subject in a camera, various methods using an imaging device applied a photoelectric converter such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) have been known. In the recording of an image of a subject according to the method, an image of a subject is formed on an imaging device through an optical system such as a zoom lens, and the light quantity of the image is converted into an electric output by a photoelectric converter to be stored in a memory medium.
With recent progress in micro fabrication, CPU (Central Processing Unit) gains more processing speed and a memory medium becomes high degree of integration. Accordingly, a large amount of image data that are yet to be handled can be processed quickly. Moreover, an imaging device also becomes high degree of integration and is miniaturized. The high integration of an imaging device makes it possible to record high spatial frequency. Miniaturizing the imaging device makes it possible for a camera body to become compact.
However, the high integration and miniaturizing of an imaging device narrow the light detecting area of each photodetector. Accordingly, electrical output falls to produce a lot of noise. In order to prevent this, the optical system is made to be faster to increase the light amount reaching the imaging device. Moreover, a minute lens element that is so-called a micro-lens-array may be arranged directly in front of each photoelectric detector.
The micro-lens-array arranged directly in front of the photoelectric detector can guide light falling between adjacent photoelectric detectors to the photoelectric detector. However, when the exit pupil of the optical system approaches the imaging device, in other words, when the angle between the principal ray incident to the imaging device and the optical axis becomes large, off-axis ray proceeding to periphery of the image makes a large angle not to reach the imaging device, so that it causes insufficient light amount. Accordingly, by means of arranging a micro-lens-array directly in front of the photoelectric detector, the light falling between adjacent photoelectric detectors can be guided to the photoelectric detector as described above, but it inevitably imposes restriction on the exit pupil position of the optical system.
A digital still camera, so called, which records an image of a subject by an imaging device using a photoelectric detector has a merit that you can easily confirm shot image without development and comfortably handle image data. On the other hand, image quality is inferior to that of a film camera and you have to connect it to an instrument such as a personal computer to process the imaging data. Therefore, the diffusion of the digital still camera has not been increased. Owing to recent progress in imaging quality and diffusion of the instrument, digital still cameras have been using generally.
In order to improve image quality, it is inevitable that optical performance of the optical system is to be increased in addition to high integration of the imaging device. Moreover, increasing the zoom ratio of the optical system provides more degree of freedom for shooting to photographers and has the following merits that they can shoot a close-up as they are further close to the subject and that they can shoot an wide area even if the subject locates closer such as an indoor scene.
Zoom lenses suitable for a camera which records an image of a subject by an imaging device using a photoelectric detector have been proposed in Japanese Patent Application Laid-Open Nos. 2001-56436 and 2001-242379. The zoom lens composed of, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power. The zoom lens is a positive-negative-positive-positive four-lens-group type zoom lens, so called. When the state of lens group positions varies from a wide-angle end state to a telephoto end state, the first lens group and the fourth lens group are fixed and the second lens group and the third lens group are moved.
Another zoom lens has been proposed in Japanese Patent Application Laid-Open No. 2001-356269. The zoom lens is also a positive-negative-positive-positive four-lens-group type zoom lens. When the state of lens group positions varies from a wide-angle end state to a telephoto end state, the first lens group, the second lens group, and the third lens group are moved.
Another zoom lens has been proposed in Japanese Patent Application Laid-Open No. 2001-188170. The zoom lens is also a positive-negative-positive-positive four-lens-group type zoom lens. When the state of lens group positions varies from a wide-angle end state to a telephoto end state, all lens groups are moved.
Another zoom lens has been proposed in Japanese Patent Application Laid-Open No. 7-5361. The zoom lens is also a positive-negative-positive-positive four-lens-group type zoom lens. When the state of lens group positions varies from a wide-angle end state to a telephoto end state, the first lens group and the third lens group are moved to the object, the second lens group is moved to the image, and the fourth lens group is moved at first to the object and then moved to the image.
Another zoom lens as an interchangeable lens for an SLR camera has been proposed in Japanese Patent Application Laid-Open No. 57-5012. The zoom lens is also a positive-negative-positive-positive four-lens-group type zoom lens. When the state of lens group positions varies from a wide-angle end state to a telephoto end state, the first lens group and the third lens group are moved to the object, the second lens group is moved at first to the image and then moved to the object, and the fourth lens group is moved at first to the object and then moved to the image.
With progress in the high integration of the imaging device, it becomes necessary that the optical system can realize high contrast with respect to high spatial frequency. At the same time, since a light detecting area of each photoelectric converter becomes small, in order to secure sufficient light quantity to each photoelectric converter, it becomes necessary that the optical system has a large aperture ratio. As a result, it causes problems such that the number of lens elements becomes large, and the optical system becomes large.
Digital cameras, as getting popularity, are being used widely. User requirements for digital cameras to improve portability, in particular compactness and lightweight, are getting stronger. At the same time, a high zoom ratio is also required.
When an optical system is made to be high zoom ratio, it tends to become large. To make an optical system be high zoom ratio tends to against to making the optical system be compact. In particular, when an optical system is planned to be high zoom ratio as well as to be compact, the refractive power of each lens group consisting of the optical system tends to become large. Accordingly, variation in off-axis aberrations tends to produce in accordance with change in the state of lens group positions or change in the angle of view, so that it is difficult to improve the optical system to have high optical performance.
When the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-56436 or 2001-242379 is applied to a digital camera, since the movable lens group is only two, the moving amount of each lens group has to become large, so the zoom lens system becomes large to prevent improvement in portability.
In the case of applying the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-356269 or 2001-188170 to a digital camera, since the first lens group is at first moved to the image and then moved to the object upon varying the state of lens group positions from the wide-angle end state to the telephoto end state, off-axis rays passing through the first lens group tend to leave away from the optical axis. Accordingly, it becomes difficult to make the lens diameter be small. Moreover, when the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-356269 is applied to a digital camera, since the third lens group is rather large, so it is difficult to improve portability. Furthermore, when the zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-188170 is applied to a digital camera, since the third lens group is composed of, in order from the object, a positive lens, and a cemented lens constructed by a positive lens cemented with a negative lens, it is difficult to correct negative spherical aberration produced in the third lens group independently, so it is difficult to satisfy both compactness and correction of the aberration.
In the zoom lens disclosed in Japanese Patent Application Laid-Open No. 7-5361, since an aperture stop is moved independently with the lens groups, an aperture moving mechanism is necessary in addition to the lens group moving mechanism, so it is not suitable for compactness and lightweight.
In the zoom lens disclosed in Japanese Patent Application Laid-Open No. 57-5012, the third lens group is too close to the fourth lens group in the wide-angle end state. Accordingly, it becomes difficult to arrange the position of the exit pupil to an appropriate position.